Steel engraving (once known as siderography) is an intaglio printing process. Steel engraving developed from copperplate engraving. Because copperplates wear out so quickly, tougher and harder materials were needed for the printing plate if many more copies were to be made. In 1819/20 Charles Heath, an Englishman, and Jacob Perkins of Massachusetts collaborated on inventing a method of engraving on steel, which was the hardest material available. The drawing is executed on a plate of soft steel (also known as mild steel, which has a low carbon content) with a graver or burin or - as in etching - the plate is bitten. Steel engraving requires much more delicate and closely set lines than copperplate engraving. It is particularly suited to making very fine, small-scale representations but cannot produce the saturated depths of a copperplate engraving. Steel engraving was used in the 19th century mainly for book illustration and as a reproduction process, especially in the making stamps and bank notes, for which it was originally developed. The virtually unlimited editions made possible by steel engraving served commercial interests. When technology advanced and more economical solutions were found, the steel engraving went virtually out of use in book illustration.